A voltage-controlled oscillator (VCO) produces a periodic time-varying output signal the frequency of which is controlled by the voltage level of an input signal. VCOs are widely used in circuits such as phase-locked loops to provide controllable periodic signals, and they are also commonly used to generate clock signals for microprocessor systems.
Recent improvements in the speed of microprocessor systems have caused an increased demand for faster clock signals with highly controllable and accurate signal attributes, such as duty cycle, mark space ratio, rise and fall times, etc. In fact, in many systems, to improve system processing speed, both the rising and falling edges of clock signals are being used to trigger events. In such systems, it is very important that the clock signals be symmetrical, i.e., that they have very precise 50-50 duty cycles. In conventional VCOs, this is very difficult to achieve because the signals are typically generated in purely digital form by digital circuitry such as inverters in ring oscillator circuits. Ring oscillator frequency is typically controlled by varying VDD. This shifts the peak-to-peak amplitude of the output of the ring oscillator such that when it is amplified to the logic swing voltage of the chip, further asymmetry in the output waveform is introduced. Because of many factors such as varying tplh and tphl ratios of the inverters, rising and falling edges of square pulses are different. The resulting square waves are typically not symmetrical and therefore cannot readily be used to control timing in these new high-speed systems.
The present invention is directed to a voltage-controlled oscillator circuit and a method of generating a square wave signal which overcome these drawbacks of the prior art. The VCO of the invention includes a ring oscillator circuit which generates a series of pulse signals. A filter circuit converts the series of pulses into a substantially sinusoidal signal. An amplifier converts the substantially sinusoidal signal into a substantially square wave signal.
In one embodiment, the ring oscillator circuit includes a plurality of inverter circuits connected in series. The supply voltage to the inverters can be varied to control the oscillation frequency.
The filter circuit receives the series of pulses from the ring oscillator circuit. It filters the pulses to convert them into a sinusoidal signal. The filter can include a low-pass filter which removes high-frequency components and tends to smooth the pulses into the sinusoidal signal. The filter can include an ac coupling capacitor which passes the ac components of the pulses and blocks the dc components of the pulses. The resulting approximate sinusoidal signal is substantially symmetrical about the reference potential of the system.
In one embodiment, the frequency of the square wave generated by the system is between 600 MHz and 1.5 GHz. In one particular embodiment, the frequency is above 1 GHz.
The VCO circuit and method of the invention provide substantial advantages over prior approaches. Because the circuit of the invention filters the output of the ring oscillator circuit, a substantially sinusoidal signal is applied to the amplifier to produce the square wave output. The symmetry of the sinusoidal signal results in a square wave signal that is also highly symmetrical and is accurate and controllable. As a result, the square wave signal can be used in the demanding high-speed processing environment.